A conventional Co-based alloy sputter target hitherto used comprises in part a hexagonal close packing (h.c.p.) phase, the remainder being a phase which is largely face centered cubic (f.c.c.) with a smaller amount of h.c.p. This conventional target is manufactured by a process wherein a Co-based alloy material comprising a single f.c.c. phase is heated until it is melted, and is then poured into a mould and is cooled from the high temperature at which it is in the f.c.c. single phase. The resulting molded alloy is allowed to cool immediately or after it is subjected to a hot-working treatment, so that part of the f.c.c. single phase is transformed into a Martensite structure which includes a h.c.p. phase. The conventional Co-based alloy thus manufactured has been used for magnetron sputtering.